1. Field of The Invention.
The present invention relates to electroplating apparatus and methods, and more particularly the methods and apparatus for the plating of printed circuit board thru-holes and the like.
2. Prior Art.
For various reasons, the proper plating of thru-holes in printed circuit boards is an important aspect of the fabrication of certain types of printed circuits. In particular, far superior mechanical and electrical connection may be made to electronic component leads if the printed circuit board through holes are suitably copper plated, as the soldered bond to the component leads will in general run the entire length of the thru-holes rather than merely the length of whatever solder maniscus there is on the soldered component lead and the face of the printed circuit board on which soldering takes place. Further, in multilayer printed circuit boards, plated through holes are used to make electrical contact with buried printed circuit layers, so that a high quality plating and appropriate copper buildup throughout the entire length of the thru-hole is critical to the accomplishment of high reliability multilayer interconnects. The achievement of such thru-hole plating however is difficult for various reasons. In particular in a conventional plating tank the edges of the mouth of a through hole on a printed circuit board cause particularly high currents and plating rates at that location, effectively depleting the ions in the electrolyte passing into the thru-holes so that the plating rate and plating quality grossly diminish along the depth of the thru-holes depending upon the aspect ratio, that is, the length to diameter ratio of the thru-holes. Consequently, high aspect ratio thru-holes cannot be repeatably and reliably plated using conventional plating methods.
In the prior U.S. Pat. No. 4,174,261, there is disclosed methods and apparatus for providing high and uniform processing rates for electroplating, deplating, etching and the like, substantially independent of the surface geometries of the article subjected to the process. In accordance with that process in an electroplating application, the article to be plated is supported on a cathode so that the electrolyte may be forceably sprayed on the article from an array of spray nozzles adjacent the surface thereof. Intermixed with the array of spray nozzles may be a second array of openings providing suction to locally remove most of the sprayed electrolyte after impingement on the workpiece. The net effect is that fresh electrolyte is constantly being sprayed onto the article being plated, including being sprayed into thru-holes of a printed circuit board, with the spent electrolyte being quickly removed from the workpiece before it has an opportunity to shield the surface thereof from the spray of fresh electrolyte. Functionally, the method and apparatus of that patent work very well, resulting in uniform, very high density plating on flat surfaces, and very good deposition distribution along relatively deep thru-holes in printed circuit boards, not generally achievable with other techniques. The process is less than optimum however from a noise and energy standpoint, as a relatively high pumping power is required to give the best results, and the process itself is relatively noisy, particularly with multiple tanks operating in a typical production environment.